Ultraviolet damage to the eye revisited: eye-sun protection factor (E-SPF®), a new ultraviolet protection label for eyewear

Abstract

Ultraviolet (UV) radiation potentially damages the skin, the immune system, and structures of the eye. A useful UV sun protection for the skin has been established. Since a remarkable body of evidence shows an association between UV radiation and damage to structures of the eye, eye protection is important, but a reliable and practical tool to assess and compare the UV-protective properties of lenses has been lacking. Among the general lay public, misconceptions on eye-sun protection have been identified. For example, sun protection is mainly ascribed to sunglasses, but less so to clear lenses. Skin malignancies in the periorbital region are frequent, but usual topical skin protection does not include the lids. Recent research utilized exact dosimetry and demonstrated relevant differences in UV burden to the eye and skin at a given ambient irradiation. Chronic UV effects on the cornea and lens are cumulative, so effective UV protection of the eyes is important for all age groups and should be used systematically. Protection of children's eyes is especially important, because UV transmittance is higher at a very young age, allowing higher levels of UV radiation to reach the crystalline lens and even the retina. Sunglasses as well as clear lenses (plano and prescription) effectively reduce transmittance of UV radiation. However, an important share of the UV burden to the eye is explained by back reflection of radiation from lenses to the eye. UV radiation incident from an angle of 135°-150° behind a lens wearer is reflected from the back side of lenses. The usual antireflective coatings considerably increase reflection of UV radiation. To provide reliable labeling of the protective potential of lenses, an eye-sun protection factor (E-SPF®) has been developed. It integrates UV transmission as well as UV reflectance of lenses. The E-SPF® compares well with established skin-sun protection factors and provides clear messages to eye health care providers and to lay consumers.

Keywords: aging; back reflection; irradiation; prevention; risk reduction; solar irradiance; transmission.

Figure 1

Spectral distribution of solar radiation and ultraviolet radiation, visible light, and infrared radiation. Depicted is the spectral distribution as measured at noon, at 40°N latitude, 20° incidence angle compared with zenith, and a 0.305 cm thickness of ozone layer.

Figure 2

Photon energy as a function of wavelength in the optical radiation spectrum. The shorter the wavelength, the higher the photon energy. Abbreviations: UV, ultraviolet; IR, infrared; UV-C, far ultraviolet; UV-B, mid ultraviolet; UV-A, near ultraviolet.

Figure 3

Hourly average of UV-B intensity in the central eye when facing towards and away from the sun (in volts). Notes: When facing towards the sun, exposure differs markedly depending on time of the year/solar altitude (red line, November/autumn; black line, September/summer). Indirect exposure is less influenced by time of the year (green and blue lines). Reprinted with permission from Sasaki H, Sakamoto X, Schnider C, et al. UV-B exposure to the eye depending on solar altitude. Eye Contact Lens. 2011;37:151–155. Copyright 2011 Wolters Kluwer Health. Promotional and commercial use of the material in print, digital or mobile device format is prohibited without the permission from the publisher Lippincott Williams & Wilkins. Please contact journalpermissions@lww.com for further information. Abbreviation: UV-B, mid ultraviolet.

Figure 4

UV transmission is blocked efficiently by most lenses, but AR increases back reflectance of UVR into the eye. Note: Violet arrows, UV radiation; yellow arrows, visible light. Abbreviations: AR, antireflective coating; Cx AR, antireflective coating of the convex lens face; Cc, concave lens face; UV, ultraviolet.

Figure 5

(A) Back side reflection measurement at 30° of a standard clear lens with multilayer antireflective coating. At 300 nm, more than 15% of UVR is reflected (green line). A lens with enhanced efficiency (pink line) reduces the back reflection in the UV range markedly (Crizal UV™, Essilor International SA, F-Charenton-le-Pont, France). (B) Reflectance characteristics for various clear high-index materials with antireflective coating and uncoated crown glass and acrylic. Reprinted from with permission Citek K. Anti-reflective coatings reflect ultraviolet radiation. Optometry. 2008;79:143–148. Copyright 2008 Elsevier. Abbreviations: UV-C, far ultraviolet; UV-B, mid ultraviolet; UV-A, near ultraviolet; UVR, ultraviolet radiation.

Figure 6

E_s(λ) is the spectral distribution of solar radiation (ambient exposure) in W/m²/nm (orange line). S(λ) is the relative spectral function efficiency (UV-B more dangerous than UV-A) in arbitrary units (blue line). Abbreviations: UV, ultraviolet; UV-B, mid ultraviolet 280–320 nm; UV-A, near ultraviolet 320–380 nm

Figure 7

Calculation of UV reflection. Note: Calculation of UV reflection according to the formula used in the International Organization for Standardization standard for UV transmission. Spectral ellipsometry and variable angle spectrophotometry in both the UV and visible range are used to characterize every material, be it substrate or thin layer. Spectral reflection measurements on concave (back side) were performed on antireflective lenses using a spectrometer (MCS501, Carl Zeiss Meditec AG, D-Jena, Germany) and a specific lens holder designed by Essilor International SA (F-Charenton-le-Pont, France) that allows measurements at different angles in the 30°–45° range, corresponding to real-life back side solar exposure for a wearer in the 135°–150° range. Abbreviation: UV, ultraviolet.

Figure 8

Location of eyelid malignancies: percentages for 174 tumors. Notes: Reprinted by permission of Mayo Foundation for Medical Education and Research. All rights reserved. Cook BE, Jr, Bartley GB. Epidemiologic characteristics and clinical course of patients with malignant eyelid tumors in an incidence cohort in Olmsted County, Minnesota. Ophthalmology. 1999;106(4):746–750. Abbreviations: BCC, basal cell carcinoma; SCC, squamous cell carcinoma; MM, malignant melanoma in the periorbital region.

Figure 9

Two extreme cases that need to be considered: UV transmission (left, yellow) when the sun is in front of the wearer, and UV back reflection (right, purple) when the sun is coming from the back of the wearer. Notes: Relevance of transmission and reflection properties depends on the solid angle of incident UVR. Incident angle, 0°, UVR (violet) is blocked efficiently by lenses. Reflection does not play a role. Incident angle, 135°–150°, with many antireflective coatings, UVR is reflected by the reverse side of the lens and refracted into the eye. This situation arises when facing away from the sun (sun almost behind the wearer). Abbreviations: UVR, ultraviolet radiation; UV, ultraviolet.